FIG. 1 shows a conventional infrastructure mode wireless local area network (WLAN) 100. The WLAN 100 includes at least one access point (AP) 102 which is associated with at least one WTRU 104. The AP 102 is responsible for servicing the communication needs of its associated WTRUs 104. The WTRU 104 sends uplink traffic to the AP 102, and the AP 102 sends downlink traffic to the WTRU 104. In an independent basic service set (IBSS), WTRUs 104 talk directly to each other in an ad-hoc manner without the need for an AP 102 to be present.
In IEEE 802.11 WLAN systems, a WTRU 104 continuously listens to, (i.e., monitors), the wireless medium to determine if there are frames being transmitted. If there are frames being transmitted on the wireless medium, the WTRU 104 receives and decodes such frames in order to determine whether such frames are destined, (i.e., addressed), to itself or not. Listening or receiving and decoding frames consume significant power of the WTRU 104 as summarized in Table 1. Table 1 describes different states of the WTRU 104 and its power consumption level in each state.
TABLE 1approximate averagepower consumptionStateGeneric description(peak consumption = 100%)Active TxDevice in the process of80%transmitting a burstActive RxDevice in the process of50%receiving a burstListenDevice actively listening to30%the mediumStandbyDevice ignoring the medium, 5%but capable of Active Tx,Active Rx, and Listen withina short time span(typically <10 μs)SleepDevice substantially turned<0.1%   off, with change time to adifferent state in the orderof >1 ms
The problem with the conventional WLAN 100 is that a WTRU 104 may spend a lot of time in a listen state only to find out that there are few or no frames being transmitted on the wireless medium, and may spend a lot of time in an active receive state only to find out that the frames it received, (i.e., demodulated and decoded), are not destined to the WTRU 104.
Conventional power saving schemes attempt to reduce the amount of time that a WTRU 104 spends in the active receive or listen states and to increase the amount of time that the WTRU 104 spends in the standby or sleep states. One example is automatic power save delivery (APSD) defined in IEEE 802.11e. IEEE 802.11e defines scheduled APSD (S-APSD) and unscheduled APSD (U-APSD).
In S-APSD, an AP 102 and a WTRU 104 agree on scheduled intervals during which the AP 102 will deliver data that is destined to the WTRU 104. Since the WTRU 104 has agreed with the AP 102 on specific time intervals for scheduling its data, the WTRU 104 may go into a sleep state, (i.e., the WTRU 104 does not listen to, receive or decode frames), during all other times except for its scheduled service interval that it agreed upon with the AP 102. Doing so provides high power savings for the WTRU 104, because the WTRU 104 can spend more time in the sleep or standby states.
However, a drawback of the S-APSD scheme is complexity and lack of flexibility. The S-APSD scheme is complex due to its pre-scheduled nature, where both the AP 102 and the WTRU 104 have to agree upon, and meet, tight timing constraints. For example, the WTRU 104 has to wake up at strict times, and the AP 102 has to schedule data for the WTRU 104 during such strict times. In addition, the S-APSD scheme is not efficiently scalable from the AP's perspective, since the AP 102 has to store the scheduling intervals that the AP 102 agreed upon with each WTRU 104. As the number of WTRUs 104 grows, the AP 102 memory requirements will also grow. Additionally, because the AP 102 cannot send the data for the WTRU 104 immediately when the medium is available, but has to wait for that WTRU's scheduled interval, a delay and delay variation may be higher when using the S-APSD scheme.
On the other hand, in a U-APSD, the WTRU 104 may sleep, and wake up on its own to send a trigger frame to the AP 102. In reaction to the trigger frame, the AP 102 may send data to the WTRU 104. Drawbacks of the U-APSD are that when a WTRU 104 wakes up to send a trigger frame, it effectively consumes more power since the active transmit state consumes the most power. In addition, the U-APSD scheme may potentially waste the wireless medium, because the AP 102 may not have any data to send to the WTRU 104 in response to the trigger frame. Another problem with the U-APSD scheme is that the WTRU 104 may cause a collision since the WTRU 104 may not detect the channel as busy.
To enhance throughput in the new IEEE 802.1 in standard, several frame aggregation mechanisms have been introduced, such as medium access control (MAC) protocol data unit (MPDU) aggregation, physical layer (PHY) PDU (PPDU) aggregation, and PPDU bursting. Since such mechanisms generally aggregate multiple frames, new mechanisms such as multiple receiver aggregate (MRA) multi-poll (MMP) and power saving aggregation descriptor (PSAD) have been proposed in order to improve power saving performance.
The basic idea underlying the MMP and PSAD is that since an aggregated frame may be quite long, instead of having a WTRU 104 receive the entire aggregated frame only to find out that it does not have data within it, the AP 102 first sends an MMP or PSAD frame to describe which WTRUs' addresses are included in the subsequent aggregated frame. The AP 102 first sends a frame to preannounce the destinations and transmission times of the upcoming sequence of data. WTRUs 104 which have data in the upcoming sequence can sleep and only start listening or receiving their data at the scheduled (pre-announced) times. WTRUs 104 that do not have data within the upcoming sequence can also save power by sleeping during the upcoming data sequence.
FIG. 2 shows a conventional MMP frame 200. The MMP frame 200 is used to define multiple response periods in combination with multiple-receiver aggregation. The MMP frame 200 includes a frame control/duration field 202, a receiver address field 204, a transmitter address field 206, an N STA field 208, receiver information fields 210 and a cyclic redundancy check (CRC) field 212. The N STA field 208 indicates the number of receivers for which MPDUs are included inside the MRA aggregate. The receiver information fields 210 indicate each receiver address in the MRA aggregate. Each receiver information field 210 includes a receive offset field 214, a receive duration field 216, a transmit offset field 218 and a transmit duration field 220. The receive offset field 214 defines the start of the first symbol containing downlink data for the WTRU relative to the start of the PPDU carrying the MMP. The receive duration field 216 defines the length of the downlink data. The transmit offset field 218 defines the time when transmissions by this WTRU may start. The transmit duration field 220 defines the limit of transmission duration.
FIG. 3 shows a conventional process for the MMP frame exchange between an AP 102 and plurality of WTRUs 104. The AP 102 sends an MMP frame 302 to the WTRUs 104. The MMP frame 302 includes the uplink and downlink transmission schedule. After a short inter-frame spacing (SIFS), the AP 102 sends downlink data to the WTRUs 104 which are scheduled by the receive offset and receive duration for each recipient WTRU 104 in the MMP frame 302. FIG. 3 shows transmission of downlink frame to five (5) WTRUs 104 which are sent in two MRA frames 304,306 separated by the SIFS. Each WTRU 104 sends an uplink transmission in a scheduled period by the transmit offset and transmit duration. The receive offset and the transmit offset are set with reference to the completion of the transmission of MMP frame 302.
FIG. 4 shows a conventional PSAD frame 400. The PSAD frame 400 includes a frame control/duration field 402, a receiver address field 404, a transmitter address field 406, a basic service set identity (BSSID) field 408, a PSAD parameter field 410, a receiver information field 412 and a CRC field 414. The PSAD parameter field 410 is used to describe power save aggregation (PSA) access phase (PAP) which immediately follows the PSAD frame 400. The duration field 402 indicates the total time duration of all of the downlink and uplink transmission opportunities (TXOPs) which are described in the receiver information fields 412. The PSAD parameter field 410 includes a descriptor end field 416, which indicates the duration of the PAP which is described by the PSAD frame 400.
According to the IEEE 802.11n joint proposal specification and the enhanced wireless consortium (EWC) specification, a power save multi-poll (PSMP) feature has been introduced. A PSMP frame is a MAC management action frame with destination address set to broadcast that provides a time schedule for downlink transmission (DLT) and uplink transmission (ULT) to be used by the PSMP transmitter and PSMP receivers. The scheduled time begins immediately subsequent to the transmission of the PSMP frame. The DLT is a period of time described by the PSMP frame and which is intended to be used for the reception of frames by PSMP receivers. The ULT is a period of time described by the PSMP frame and which is intended to be used for the transmission of frames by a PSMP receiver.
FIGS. 5A-5C show a format of the PSMP frame 500, a PSMP parameter set field 502 of the PSMP frame 500 and STA Info field 504 of the PSMP frame 500. The PSMP parameter set field 502 includes N_STA field 506 and a PSMP sequence duration field 508. The N_STA field 506 indicates the number of STA Info fields present. The PSMP sequence duration field 508 indicates the duration of the current PSMP exchange which is described by the PSMP frame relative to the end of the PSMP frame. Each STA info field 504 includes a DLT start offset field 510, a DLT duration field 512, a ULT start offset field 514 and a ULT duration field 516 to schedule the DLT and the ULT.
The MMP, PSAD and PSMP schemes achieve their power savings basically by converting most of the time that WTRUs 104 unnecessarily spend in the active receive state into time in either the sleep or standby states, through pre-identifying which WTRUs or groups of WTRUs are targeted by the upcoming transmissions on the wireless medium.
The drawbacks with the MMP, PSAD and PSMP schemes are that they require some form of pre-scheduling, albeit on a smaller scale and more short-term than IEEE 802.11e APSD scheduling. MMP, PSAD and PSMP frames also have significant overhead, and hence are not very efficient in using the wireless medium. The MMP, PSAD and PSMP schemes provide power savings only when there are frames being transmitted, (i.e., when the wireless medium is being utilized), and do not provide power savings when the wireless medium is idle.